The present invention relates to electrical circuits for providing current through a load impedance, where the current source is differential and provides a symmetrical, high impedance drive to the load. This type of current source is particularly useful in instrumentation that makes impedance measurements using low excitation.
Many different types of current sources are known in the art. These circuits vary greatly in complexity and performance to fit their intended application. Furthermore, they are frequently modified to support unipolar or bipolar and fixed or voltage programmable operation. Instrumentation current sources generally fall into two categories, those that drive floating loads and those that drive grounded loads. The single op-amp, ideal current source (see, e.g., p.182 xe2x80x9cThe Art of Electronicsxe2x80x9d, Horowitz and Hill, Cambridge University Press, 1989) is an example of a current source that drives a floating load. One problem with this type of source is that the output current is not controlled if the load is grounded to the current source common. The ideal source also produces a common mode voltage on the load that may be unwanted.
A modified Howland current source, illustrated in FIG. 1, is an example of a current source that will drive a grounded load. It was developed specifically to drive current into a load that has one end tied to the current source ground, ensuring low impedance between the load and ground. The Howland current source with its many variations and improvements has been used very successfully in instrumentation, but it does have one drawback. The current source output and return have different impedances, creating an unbalanced load. Different impedances at each end of a current source load do not create a problem for the current source in isolation, but in application there is always some stray noise coupling into the load. Most noise couples either capacitively or inductively from the environment into the current source leads. Noise can also couple through the voltage measurement circuits via their power supplies. Much of the noise couples into the load as common-mode current. Differential voltage inputs are used extensively to reject common mode voltage noise but they too are limited.
When common mode currents act on an unbalanced load, they react differently with different impedances on each side of the load to create normal mode errors that cannot be rejected by differential voltage inputs. These errors can be significant when making precision measurements or whenever the common mode voltages are large compared to the desired measurement signal. One method for balancing the load is to provide a differential current source with a symmetrical, high impedance drive to the load. An early example of a differential current drive is the circuit described in NASA Tech Brief #MSC-16475, Winter 1977. This circuit provides differential output with two identical, out of phase outputs, but drives one end of the load from zero source impedance. This effectively introduces a ground at one end of the load and will not eliminate common mode voltage errors. A second example of a differential current source, illustrated in FIG. 2, is the circuit described in U.S. Pat. No. 5,021,729. This differential current source is a practical circuit and achieves the goal of minimizing the effect of common mode voltage on precision measurements as long as the load remains floating. However, if one side of the load is grounded to the current source common there is a significant error in current output.
Thus there is a need for a new current source that has the advantages of a differential current source for floating loads but will also tolerate grounded loads.
The primary object of the present invention is to provide a new current source that operates as a differential current source when the load is floating and, upon grounding the load, will automatically operate as a grounded load current source with reduced common mode rejection.
Another objective is to ensure the load current will change only by a small, predictable amount when operation changes from one mode to another, thereby enabling the invention to be used in applications where the measurement wiring is pre-installed and difficult to change or when unintentional grounding occurs during operation.
Another objective is to create a current source that can be configured for an operation selected from the group consisting of AC operation, DC operation, unipolar operation, bipolar operation, for operation with resistive loads, for operation with reactive loads, and a combination thereof.
Another objective is that the invention can be easily modified for range selection by switching in sense resistors of different values.
To achieve these objectives, the present invention is constructed of two, substantially identical, modified Howland current sources driving the load simultaneously but out of phase with each other. The current from each half is equal to the full desired current output but they have opposite sign. An external voltage source drives one half of the current source and an amplifier is used to create an opposite phase voltage to drive the other half. The inverting amplifier can optionally be omitted if the second half of the current source is configured for inverting operation. In addition to the Howland architecture, the invention requires active feedback, to center its operation about ground. This prevents the current source output amplifiers from floating to one power supply rail or the other (e.g., active feedback comprising a means to center the operating voltage between power supply rails). The circuits used to center operation also act to reduce DC current if the current source is configured for AC operation.
The near perfect symmetry of the circuit according to the present invention allows one half to operate essentially alone if the other is grounded. Any impedance between zero and infinity can be tolerated between one half and ground, allowing for significant lead resistance or high impedance shorts between one side of the load and ground. The error in current between floating and grounded operation is limited to one half of the difference between the two halves. The current into a floating load is the average of the current that would be produced by the two halves if they were standalone. The current into a grounded load is that of the ungrounded half. This difference can be very small if a precision voltage inverter and well-matched programming resistors are used. If the load is floating, active common mode reduction can be added that uses active feedback to reduce noise induced common mode voltage on the load. Common mode voltage is sensed and fed back to intentionally bias the operating point.
When tracking common mode voltage, the current source can cancel much of its effect by creating low impedance to the common mode voltage while maintaining its typically high impedance to the load.
These and other objectives and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.